This invention relates to a metallic tone coating composition which is excellent in stability and capable of affording a coating film having good durability.
Hitherto, a variety of metal powders such as powders of aluminum, copper, iron and various alloys have been used for a metallic tone coating composition. Among them, aluminum powder is used most frequently. In the case of using aluminum powder, the surface thereof is usually protected by coating a fatty acid such as oleic acid or stearic acid to prevent the aluminum powder from causing a dust explosion which may occur when uncoated powder is scattered. However, a metallic tone coating composition prepared by using aluminum powder treated with a fatty acid, involves several problems; the treated aluminum powder in the coating composition is liable to sediment and once sedimented, is difficult to redisperse. Depending on the type of coating resin used, aluminum ions which are produced by dissolution of aluminum tend to react with the resin, resulting in increased viscosity or gelatin of the paint composition. Such a paint composition thus lacks stability as required for a useful paint. Metallic tone coating compositions prepared by using other kinds of metal powder such as copper powder, also have the same defect in the coating, composition containing aluminum powder; viz. the copper powder is liable to sediment and once sedimented, the powder in the coating composition changes the color of the composition or reacts with the resin, thus causing gelation of the coating composition.
When using powders of an alloy consisting of metals with different ionization tendencies or a mixture of at least two different kinds of metal powder, ionization of the metal is significantly promoted so that the coating composition increases in viscosity and gelation of the coating composition becomes conspicuous. Such coating compositions also have the defects that the color of the metal powder is changed due to the action of an alkali or acid present in the resin and when packed in an air-tight container for transportation, the container will be deformed due to the inner pressure of hydrogen gas evolved therein. In addition, a coating film obtained by using a conventional metallic tone paint is insufficient in resistance against water and chemicals such as acids and alkalis. Especially if the coating compositions are used outdoors, the higher the content of the metal, the quicker the luster will be lost.
As a result of much research carried out in order to overcome such drawbacks of conventional metallic tone paints, it has now been found that the afore-mentioned drawbacks can be overcome by preparing a metallic tone coating composition which contains a specific amount of a metal powder, the surface of which has been treated in a special manner. This invention is based on this finding.